Generally, these systems associate an MWIR (3 μm-5 μm) imaging band with a visible (0.4 μm-0.7 μm) and/or NIR (0.7 μm-1.1 μm) and/or SWIR (1.4 μm-2.4 μm) imaging band, and laser channels, typically at 1.06 μm and/or 1.54 μm.
In such systems, it is naturally necessary to split spectrally the various channels using one or more spectral splitting components. It is a question here of the first splitting stage that for example transmits the MWIR band and reflects the other bands, the other splitting stages concerning the splitting of the other bands reflected by this first stage.
The only viable solution consists in transmitting the MWIR band and reflecting shorter wavelengths. This is because the solution consisting in reflecting the MWIR and transmitting the shorter wavelengths is reputed to be unfeasible, or at least particularly difficult to implement on account of the complexity of the thin-film stack allowing this function to be provided.
Splitting using a dichroic plate into parallel beams downstream of an afocal frontal system would appear at first glance to be advantageous because it does not produce any parasitic images or aberrations. In contrast, it requires many multispectral components upstream of the splitter in order to form the frontal afocal system, this more than offsetting its advantages.
To guarantee the optics retain a satisfactory degree of compactness and to optimize their transmission in each of the channels, it may be more judicious to split the incident beam into beams that converge, just downstream of the focal point of the multispectral head objective. The upstream-downstream direction is that of the light propagating in the system.
In summary, a spectral splitting component is sought that:                transmits the MWIR band and reflects bands of shorter wavelengths: visible (0.4 μm-0.7 μm) and/or NIR (0.7 μm-1.1 μm) and/or SWIR (1.4 μm-2.4 μm);        works with convergent beams;        does not degrade the modulation transfer function (MTF), either in transmission or in reflection; and        does not create a visible parasitic image, in particular in the MWIR band.        
Dichroic plates and sets of splitter prisms are known techniques for splitting convergent beams, and these techniques will be described in succession.
A dichroic plate 1 is used in a system at an angle of inclination to its optical axis Oz, this angle of inclination typically being between 30° and 45° as shown in FIGS. 1a and 1b. The optical axis in fact follows a zigzag line and comprises an optical axis Oz incident on the plate and an optical axis refracted by the plate.
The plate is generally made of silicon (Si), germanium (Ge), zinc sulfide (ZnS) or zinc selenide (ZnSe).
The dichroic front face reflects the short portion of the spectrum (visible or NIR or SWIR) and transmits the long portion (MWIR) as illustrated in FIG. 1a by the arrows; the back face is simply given an MWIR anti-reflection treatment. It will furthermore be noted that in the intermediate focal plane 2, the plate clearly shifts the useful beams downward relative to the incident optical axis.
If the plate has planar and parallel faces, the two internal reflections produce a parasitic MWIR image that is slightly unfocused (focused on the plane 2′ instead of on the plane 2) and shifted considerably upward relative to the main image, as shown in FIG. 1b, in which the useful beams of FIG. 1a are not shown in order to avoid cluttering the figure. Thus, it will be understood that a poorly contrasted zone of interest located in the scene in the vicinity of a bright object may be masked by the parasitic image of this object.
By comparing the two preceding figures, it may be seen that the plate separates the useful beams from the parasitic beams. By thinning the component, the parasitic image is brought closer to the main image, which is a good thing; however, it is necessary to preserve a certain thickness in order to keep deformation of the component (notably during operation) within acceptable limits, and the advantage gained remains insufficient.
Furthermore, the inclination of the plate to the incident beam produces aberrations, namely coma and astigmatism, in the transmitted wavefront; the severity of these two effects increases with the thickness of the plate. By making the plate slightly prismatic and correctly orienting the prism to the beam, i.e. by arranging the plate so that, relative to the incident optical axis Oz, the back face of the plate is more inclined than the front face, it is possible to decrease substantially—or even considerably—these aberrations. Unfortunately, the orientation of the prism allowing the aberrations to be decreased further separates the parasitic image from the main image, relative to a simple plate with planar and parallel faces, thereby making it more visible, and thus more problematic. This is illustrated in FIG. 1c for a 1.7 mm-thick dichroic plate made of YAG, working at F/4.0, this plate being inclined at 36° to the optical axis of a system equipped with this plate. The weighted polychromatic MTF calculated in transmission in the band 3.4 μm-4.2 μm in the radial direction (R) and in the tangential direction (T) is practically diffraction limited, as shown in FIG. 1d. In order not to clutter this figure, only the response at the centre of the field has been shown, but it is easy to show that the MTF remains uniform in a field of 12 mm diameter, compatible with use with a matrix-array detector.
A simplified version having two outputs of the splitter prism assembly described in FIG. 3 of U.S. Pat. No. 3,202,039 allows the need described above to be met. As an adhesive that is sufficiently transparent in the MWIR band does not exist, the two constituent prisms must be separated by a thin air gap. Furthermore, the inclination of the beams to the internal dichroic surface means that the two prisms must be made from an optical material of low refractive index—typically lower than 1.7—in order to prevent any total reflection of the MWIR beams in this location. Given the envisioned useful spectral bands, this condition greatly restricts the choice of materials. In practice, only fluorine (═CaF2), which has an index of about 1.4, but is fragile and difficult to implement, may be used for this application. Furthermore, as this material has a high coefficient of expansion, deposition of the complex stack of thin films required for the dichroic function is subject to very variable production yields. Lastly, the assembly of the two prisms is particularly difficult. In brief, this solution is not truly an industrial solution and is furthermore very expensive.
Other solutions have been developed, notably those described in the following patents.
U.S. Pat. No. 4,412,723 describes an aberration corrector placed behind a plate with planar and parallel faces. But it does not suppress parasitic images.
U.S. Pat. No. 4,541,688 describes a device that meets the need for linear-array type detectors, but is absolutely not usable with matrix arrays; specifically, in that patent the problem addressed is solved by increasing the thickness of the plate so as to make the parasitic image depart from the field of the linear array, and the aberrations introduced by this extra thickness are corrected using additional components. To achieve the same result with a matrix-array detector, it would be necessary to increase inordinately the thickness of the component, which would make it more difficult or even impossible to correct the resulting aberrations.
U.S. Pat. No. 7,502,117 B1 describes a two-wave Michelson-type interferometer, the splitter and compensator of which comprise at least one non-planar surface in order to destroy interference between the various parasitic waves produced by the two components. The compensator and the splitter have the same power and are not prismatic.
EP 1 083 554 B1/U.S. Pat. No. 6,611,383 B1 describes correction of astigmatism using a prismatic plate, but parasitic images are not suppressed.